Removal of sulfur oxides (SOx), nitrogen oxides (NOx) and particulate matter (PM) from process gas is critical to meeting increasingly tight environmental regulations for multiple industries.
New regulations are pending for industrial process gases as well as exhaust gases. Some regulating bodies, such as US EPA, take a tiered approach to implementing new regulations on emissions from diesel engines. Marine diesel engines are targeted by new global regulations with respect to SOx and NOx.
Removal of SOx, NOx, and PM is difficult from the process engineering point of view for the following reasons:                (a) Most existing technologies only deal with one pollutant (SOx, NOx, or PM) for a given process.        (b) In many cases, the conditions that favor the removal of one pollutant, are unfavorable for the removal of another. For instance, engine technologies that reduce NOx emissions can increase the emissions of PM. Other technologies remove a single pollutant while leaving the others largely untouched.        (c) Existing technologies are expensive due to requirements for special chemical addition, expensive fuels, and combination of multiple technologies to achieve the goals of pollution control.        (d) The process conditions suitable for catalytic oxidation of SO2 and NO are not compatible. While SO2 catalysts require high temperatures (typically in excess of 380° C.) to perform adequately, catalytic NO oxidation is equilibrium-limited at high temperatures.        (e) The process conditions suitable for absorption/condensation of SO3 and H2SO4 vapors while producing concentrated acids are not adequate for absorption of NOx.        (f) Catalytic oxidation of SOx and NOx is a challenge due to the presence of PM that may foul or deactivate the catalysts.        (g) Catalytic oxidation of NOx requires attention to the load of sulfur species, which can deactivate the catalyst.        
A number of methods exist to meet the environmental regulations on SOx, NOx and PM. Typically, these methods rely on one of the following methodologies:
Use of Low-Sulfur Fuel. The use of low-sulfur fuels allows for a reduction in SOx emissions. However, low sulfur fuel is much more expensive than conventional high-sulfur fuel. Furthermore, the low-sulfur fuel alone does not reduce emissions of NOx or PM. Some embodiments of the present invention do not require changes to the fuel and reduce emissions of NOx and PM.
Gas Recirculation. NOx emissions from combustion processes can be reduced by recirculating a portion of the combustion product gas to the reaction zone to reduce the reaction temperature and oxygen content in the reaction zone, the two operating conditions that have the biggest impact on thermal-NOx formation. Gas recirculation may be known by different names, e.g. flue gas recirculation (FGR) for some combustion equipment and exhaust gas recirculation (EGR) for engine applications. However, gas recirculation does not reduce SOx emissions, and may increase PM emissions.
Engine Modifications. For engine applications, some pollution reduction systems require modifications of the engine itself, improved control of fuel injection timing and/or improved quality of atomization. Such modifications can improve the emissions of NOx and PM to a limited extent; however, in some cases, with significant fuel penalty. Further, these modifications do not reduce the emissions of SOx. Some embodiments of the present invention do not require modifications to the engine/generator and reduce emissions of SOx. In some embodiments the engine can be tuned to operate in its maximum efficiency settings.
Scrubbing of Exhaust Gas. Exhaust gas scrubbing systems, also known as scrubbers, including wet and dry scrubbers, can remove more than 90% SOx from exhaust gas, using seawater or freshwater as the scrubbing medium. The efficiency of scrubbers for removing PM, including solids and organic hydrocarbons, from diesel engines is rather poor compared to SOx removal, usually not exceeding 75%. Open-loop scrubbers are subject to several restrictions due to concerns about discharge of PAH and nitrates. Closed-loop scrubbers, on the other hand, may suffer PM and/or ash accumulation and fouling problems.
Chemical Scrubbing Technologies. These systems remove sulfur species by contacting the process gas with a chemical in a scrubber. These methods utilize a number of chemicals such as liquid caustic solutions or calcium hydroxide granules to capture the sulfur species. These methods typically produce a liquid or solid waste stream. Dry scrubbers are one example of chemical scrubbing. Depending on applications, dry scrubbers may operate over a broad temperature range. Chemical scrubbing can remove approximately 98% of SOx and approximately 50% of PM. However, these systems do not significantly reduce NOx, and the reduction in PM is typically insufficient to ensure compliance with emission control limits. Some embodiments of the present invention fundamentally differ from chemical scrubbing technologies because they do not require any additional chemical feeds, do not produce any byproducts or waste, and reduce emissions of NOx.
Absorption/Desorption Technologies. These technologies remove sulfur species using absorption/desorption columns. These methods typically utilize a fluid such as an amine to separate SO2 from tail gas. These methods require large capital investments in very tall absorption/desorption columns and consume large amounts of energy to regenerate the absorber. Moreover, these technologies, would require further handling of the recovered SO2 product, and do not reduce emissions of PM and NOx.
Selective Catalytic Reduction (SCR). SCR technologies can be effective in removing NOx from process gases. For example, urea-SCR requires the addition of urea/ammonia into the process gas to convert NOx into nitrogen gas. HC-SCR systems that use light hydrocarbons are still in the development stage. Neither type of SCR reduces emissions of PM and SOx. Further, the presence of sulfur species can deactivate the catalyst. Some embodiments of the present invention reduce emissions of PM and SOx and do not require urea/ammonia addition.
Lean NOx Traps (LNT). An LNT is an adsorber catalyst that oxidizes NO into NO2 and adsorbs it in the washcoat under lean conditions in the presence of free oxygen. When the amount of NO2 stored in the catalyst approaches its capacity, the NOx trap must be regenerated. To regenerate a NOx trap, reducing agents, such as gases containing H2, CO, C2H2, C2H4 and C3H6, are introduced to the process stream to create a reducing atmosphere, under which the adsorbed NO2 is released into the gas phase as NO, then chemically reduced by the reducing gases into N2. The use of metal oxides in the formulation often makes NOx traps vulnerable to sulfur poisoning and deactivation. The sulfated and deactivated NOx trap must be desulfated at a higher temperature to recover its adsorption capacity. Some embodiments of the present invention do not use NOx traps and reduce emissions of PM and SOx.
Dry Contact Acid Plant Technologies. If the process gas contains a high concentration of sulfur species and a low concentration of water, it can be treated using a drying tower and a conventional dry contact acid plant. The key limitations of these technologies are the inability to treat process gas streams containing low SO2 concentrations (typically about 6% mol SO2 is the limit), the inability to treat process gas streams containing high water content, and the added complexity of a drying tower acid loop. Some embodiments of the present invention do not use a drying tower and can handle the conditions of low SO2 concentrations.
WSA technology. Wet-gas sulfuric acid (WSA) technologies can be used to remove SOx in wet environments. However, these technologies do not reduce emissions of PM and NOx. Some embodiments of the present invention do not require glass tube refluxing acid condensers and reduce emissions of PM and NOx.
Diesel Particulate Filters (DPF). DPFs are wall-flow monolith filters commonly used in the automotive industry and stationary power generating units for capturing PM. However, DPFs are typically employed for use with specific process gases such as exhaust gases from the combustion of low-sulfur fuels. Some embodiments of the present invention may use a DPF as an option for PM removal, but the function can be performed by alternative means. Moreover, some embodiments of the present invention reduce emissions of SOx and NOx.